This section provides background information related to the present disclosure which is not necessarily prior art.
The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. The electric field is generated based on RF power signals generated by a radio frequency (RF) generator of a RF power system. The RF power signals generated by the RF generator must be precisely controlled to effectively execute plasma etching.
A RF power system may include a RF generator or supply, a matching network, and a load (e.g., a plasma chamber). The RF generator generates RF power signals, which are received at the matching network. The matching network matches an input impedance of the matching network to a characteristic impedance of a transmission line between the RF generator and the matching network. This impedance matching aids in maximizing an amount of power forwarded to the matching network (“forward power”) and minimizing an amount of power reflected back from the matching network to the RF generator (“reverse power”). Forward power may be maximized and reverse power may be minimized when the input impedance of the matching network matches the characteristic impedance of the transmission line.
In a typical RF power generator configuration, output power applied to the load is determined by using sensors that measure the forward and reflected power or the voltage and current of the RF signal applied to the load. Either set of these signals is analyzed to determine the parameters of the power applied to the load. The parameters can include, for example, voltage, current, frequency, and phase. The analysis typically determines a power value which is used to adjust the output of the RF power supply in order to vary the power applied to the load. In a RF power delivery system, where the load is a plasma chamber, the varying impedance of the load causes a corresponding varying power applied to the load, as applied power is in part a function of the impedance of the load. Therefore, the varying impedance can necessitate varying the parameters of the power applied to the load in order to maintain optimum application of power from the RF power supply to the load.
In the RF power generator or supply field, there are typically two approaches to applying the RF signal to the load. A first, more traditional approach is to apply a continuous wave signal to the load. In a continuous wave mode, the continuous wave signal is typically a sinusoidal wave that is output continuously by the power source to the load. In the continuous wave approach, the RF signal assumes a sinusoidal output, and the amplitude and/or frequency of the sinusoidal wave can be varied in order to vary the output power applied to the load.
A second approach to applying the RF signal to the load involves pulsing the RF signal, rather than applying a continuous wave signal to the load. In a pulse mode of operation, a RF sinusoidal signal is modulated by a modulation signal in order to define an envelope for the modulated sinusoidal signal. In a conventional pulse modulation scheme, the RF sinusoidal signal typically is output at a constant frequency and amplitude. Power delivered to the load is varied by varying the modulation signal, rather than varying the sinusoidal, RF signal.
In plasma systems, power is typically delivered in one of two configurations. In a first configuration, the power is capacitively coupled to the plasma chamber. Such systems are referred to as capacitively coupled plasma (CCP) systems. In a second configuration, the power is inductively coupled to the plasma chamber. Such systems are typically referred to as inductively coupled plasma (ICP) systems. Plasma delivery systems typically include a bias and a source that apply respective bias power and source power to one or a plurality of electrodes. The source power typically generates a plasma within the plasma chamber, and the bias power tunes the plasma to an energy relative to the bias RF power supply. The bias and the source may share the same electrode or may use separate electrodes, in accordance with various design considerations.
RF plasma processing systems include components for plasma generation and control. One such component is referred to as a plasma chamber or reactor. A typical plasma chamber or reactor utilized in RF plasma processing systems, such as by way of example, for thin-film manufacturing, utilizes a dual frequency system. One frequency (the source) of the dual frequency system controls the generation of the plasma, and the other frequency (the bias) of the dual frequency system controls ion energy.
By way of one non-limiting example, reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is typically characterized as dry etching. RIE uses a chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure (vacuum) by an electromagnetic field. High-energy ions from the plasma bombard the wafer surface and react with it to affect the etching process. In one example of a RIE system, a high frequency source RF power generator (for example, 13 MHz-100 MHz) creates a plasma, and a lower frequency bias RF generator (100 kHz-13 MHz) accelerates positive ions from the plasma to the substrate surface to control ion energy and etch anisotropy. In this example dual frequency drive system, the low frequency bias source introduces fluctuations in both the power and load impedance and into source RF generator.
One approach responsive to fluctuations in both the power and load impedance introduced into the source RF generator utilizes multiple source and bias generators to improve control of the plasma. In such a configuration, the plasma consists of a generally neutrally charged bulk region and a sheath region that oscillates near the surfaces of the vacuum chamber and substrate. The thickness of the sheath determines a significant portion of the plasma capacitance and is most affected by the low frequency bias power supply. The higher frequency source generator can be adversely affected by the sheath capacitance variation, resulting in large impedance and reflected power fluctuations. These fluctuations are usually too fast to be measured by present sensors and metrology systems.
As a result of the bias-induced capacitance fluctuations, little or no RF source power is delivered to the plasma when the reflected power is high. Conventional techniques address this limitation by increasing the power level of the source RF generator. Such a response carries with it significant control complexities and additional capital and operating costs. For example, when a RF source operates at an increased power, increased electrical stresses and a greater numbers of parts required to supply the higher power result in lower reliability of the RF generator. Further yet, such an approach impedes process reliability, as process repeatability and chamber matching are adversely affected because of parameters that cannot be reliably controlled, such as chamber RF parasitic impedances and RF amplifier component tolerances.